It has long been known that the effective resistivity of certain metals was sometimes substantially eliminated when the metal was exposed to low temperature conditions. Of particular interest were the metals and metal oxides which can conduct electricity under certain low temperature conditions with virtually no resistance. These have become known as superconductors. Certain metals, for example, are known to be superconductive when cooled to about 4.degree. on the Kelvin scale (.degree.K.), and certain niobium alloys are known to be superconductive at about 15.degree. K., some as high as about 23.degree. K.
Discovery of superconductivity in the system La-Ba-Cu-O (J. G. Bednorz and K. A. Muller, Zeit. Phys. B 64, 189-193 [1986]) has stimulated the search for other systems, particularly with a view to substituting other elements for the rare earths (RE) used in the earlier materials. For example, replacement of RE by Bi and T1 has been reported. (See M. A. Subramanian et al., Science, 239, p. 1015 (1988); L. Gao et al., Nature, 332, pp. 623-624 (1988). In preparing the system T1-Ba-Cu-O, Z. Z. Sheng and A. M. Hermann "Superconductivity in the Rare Earth-Free T1-Ba-Cu-O System above Liquid Nitrogen Temperature," Nature, 332, pp. 55-58 (1988), first mixed and ground BaCO.sub.3 and CuO to obtain a product which they heated, then intermittently reground to obtain a uniform black Ba-Cu-Oxide powder, which was then mixed with T1.sub.2 O.sub.3, ground, and heated, with formation of a superconducting material. It was noted that the T1 oxide partially melted and partially vaporized.
The superconductor system T1-Ca-Ba-Cu-O was also reported in a paper by Sheng and Hermann, "Bulk Superconductivity at 120 K in the T1-Ca-Ba-Cu-O System," Nature, 332, pp. 138-139 (1988). The authors reported "stable and reproducible bulk superconductivity above 120 K with zero resistance above 100 K". According to the paper the composition was prepared by mixing and grinding together T1.sub.2 O.sub.3, CaO and BaCu.sub.3 O.sub.4. The ground mixture was pressed into a pellet and heated in flowing oxygen. The result was cooled and found to be superconducting.
Our invention is an improvement in the latter Sheng-Hermann process of making T1-Ca-Ba-Cu-O superconductors.
See also the paper by Hazen et al, "100 K Superconducting Phases in the T1-Ca-Ba-Cu-O System," Phys. Rev. Let., 60, pp. 1657-1660 (1988}, which refers to two superconducting phases, T1 .sub.2 Ca.sub.2 Ba.sub.2 Cu.sub.3 O.sub.10+ and T1.sub.2 Ca.sub.1 Ba.sub.2 Cu.sub.2 O.sub.8+, both with onset T.sub.c near 120 K and zero resistivity at 100 K. Preparation included grinding together T1.sub.2 O.sub.3, CaO, and BaCu.sub.3 O.sub.4 (or Ba.sub.2 Cu.sub.3 O.sub.5), followed by heating.
And see "Nota Bene" in High T.sub.c Update, vol. 2, No. 6, p. 1, Mar. 15, 1988, further re properties of the T1-Ca-Ba-Cu-O system.
Wang et al., Comparison of Carbonate, Citrate, and Oxalate Chemical Routes to the High-T.sub.c Metal Oxide Superconductors La.sub.2-x Sr.sub.x CuO.sub.4, Inorg. Chem. 26, 1474-1476 (1987) discloses a carbonate precipitation technique. The precipitant was K.sub.2 CO.sub.3. According to the paper, it was necessary to wash the precipitate repeatedly, an obvious disadvantage in production work. Washing was necessary because potassium adversely affects superconductivity properties of the finished materials. If we wash repeatedly, we remove barium, a highly detrimental loss in our process.
From the technical viewpoint it may seen obvious that co-precipitated carbonates would provide enhanced homogeneity. However, the technical solution to the problem encounters serious difficulties. Thus the Wang et al process, using potassium carbonate (or sodium carbonate) necessitated numerous washings and apparently left detectable amounts of alkali in the ceramic base even so. As noted serial washings remove Ba, and would be unworkable in our process. Nor is it merely sufficient that the carbonate be derived from a cation that would burn off completely. For example, ammonium carbonate does not work, because a pH below 7 is required to prevent formation of copper tetraammine, but under these conditions bicarbonate ion is formed, with consequent formation of barium bicarbonate, which, being slightly soluble, disrupts the desired stoichiometry. Quaternary ammonium carbonates, on the other hand, form the desired metal carbonates simply and cleanly as a coating on T1.sub.2 O.sub.3 particles without troublesome side-formation of complexes or coordination compounds, with firm and precise retention of the intended stoichiometry. The coated particles are readily recovered for further processing.